There are no requirements for balustrades and handrails that are 125mm or more away from the building, so any timber that is suitable for exposure to the weather could be used. However, stair treads in BAL-FZ must be of non-combustible material, fibre-cement sheet, or a system that complies with  Australian Standard 1530.8.2, Methods for fire tests on building materials, components and structures. We are unaware of any timber stair system that has passed the test specified in AS 1530.8.2.

The 2009 edition of AS 3959 didn't deal with verandah posts. It was illogical to provide for a verandah roof, but not the posts that support it, so clause 5.7.5 was added in the 2018 edition covering verandah posts in BAL-12.5 (clause 6.7.5 in BAL-19). The wording is somewhat confusing. The clause requires posts to be mounted on stirrups not less than 75mm above adjacent ground level, or to be made from certain timbers if less than 400mm from the surface of a deck or ground. The reference to posts "less than 400mm....from the surface of the deck or ground" seems to be a drafting error.